Illumination systems have a variety of applications, including projection displays, backlights for liquid crystal displays (LCDs) and others. Projection systems usually include a source of light, illumination optics, an image-forming device, projection optics and a projection screen. The illumination optics collect the light generated by the light source and direct the collected light to one or more image-forming devices. The image-forming device(s), controlled by an electronically conditioned and processed digital video signal, produces an image light beam corresponding to the video signal. Projection optics magnify the image light beam and project it to the projection screen.
White light sources, such as arc lamps, have been, and still are, the predominant light sources used for projection display systems. In a three-panel projection system that uses a white light source, the light is split into three color channels, commonly red, green and blue and directed to respective panels that produce the image for each color. Other projection systems use only a single imager panel, and so rotating color wheels are commonly used to filter the white light so that only light from one color band is incident at the panel at one time. The light incident at the panel changes color sequentially red/green/blue/red and so on, and the panel is sequentially driven to form red/green/blue/red images synchronously with the incident light.
More recently, however, light emitting diodes (LEDs) have been considered as an alternative to white light sources. For a three-panel system, an array of red LEDs is used to illuminate the red channel, an array of green LEDs is used to illuminate the green channel and an array of blue LEDs is used to illuminate the blue channel. Some advantages of LED light sources include longer lifetime, higher efficiency and superior thermal characteristics.
LED-based illumination sources, however, display a color gamut that does not conform to recommended practices set forth by the Society of Motion Picture and Television Engineers in Recommended Practices 145-1999, “C Color Monitor Colorimetry” (referred to hereafter as SMPTS 145-1999). Also, in order to achieve an acceptable white color, LED-based illumination sources are often run in a mode that fails to realize the full intensity available from all the available LEDs, and so the overall brightness of the displayed image is reduced.
There remains a need to improve the color and brightness of LED-based illumination sources for projection systems.